Encapsulation materials have been utilized for many years to protect electronic components from mechanical stresses, humidity and other potentially threatening conditions. Such encapsulants are commonly used in electronic devices containing silicon chips, boards, and connection wires. The encapsulant is usually applied to the component in a fluid state and cured via exposure to ultraviolet or visible light, and/or by heating.
Two methods of deposition of the encapsulation material are commonly utilized. The first method is the glob top method in which the encapsulant is deposited directly on top of the electronic component and allowed to cure such that the entire component is protected. The second method is the dam and fill method. In the dam and fill method a more thixotropic material (the dam material) is deposited around the perimeter of an electronic component to be protected to create a barrier. The dam material remains immobile after deposition. A less thixotropic, more liquid material (the fill material) is placed over the electronic component within the barrier created by the dam material. The fill material preferably has a very high flow so that it can protect dies with very small pitch sizes. Both the dam and fill material may be cured via exposure to ultraviolet or visible light, through heating, or by a combination of any of these methods. Despite the desired differences in properties, currently commercially available encapsulants provide similar formulations for both the dam and fill material.
Several properties are particular important in providing an encapsulant material. Foremost, it is useful that the encapsulant have a high Tg. The thermal expansion of acrylates, epoxies and other resin systems is greatly increased at temperatures above the Tg. The thermal expansion is relatively low at temperatures under the Tg. High thermal expansion results in part failure during temperature cycling. In addition, it is important that the encapsulant not be tacky after curing. Further, as shrinkage causes stress after cure, it is desirable that the encapsulant has minimal shrinkage during cure. Finally, the rheology of the encapsulant should be such that the encapsulant is easily dispensed and stable for as long a time as possible, preferably at least six months.
It would be advantageous to provide encapsulant materials having the properties set forth above for both the dam and the fill components of the encapsulant. These materials would have the different properties desired for each specific application.